1. Field of the Invention
The invention concerns a device for identifying tubes known as straws for cryogenic storage of biological liquids, such as viral cultures, and including an envelope formed by a section of appropriate transparent material tube of constant diameter in the main part with its ends flattened to form welded seals wider than the diameter of the main part.
2. Description of the Prior Art
Patent document FR-A-2 651 793 describes a tube called a straw for cryogenic storage of biological samples, such as viral cultures, formed by a section of constant diameter tubular envelope made from a biologically neutral substantially transparent polymer material sealed at each end and including near a first end a sliding stopper comprising an aqueous gel between two porous elastic material plugs, distinguished from the prior art in that the polymer material is an ionomer resin and the seals at the ends are formed by welding the tube over a specific axial length.
By virtue of the physical properties of ionomer resins, including their thermoplastic behavior at room temperature and higher temperatures, allowing an excellent seal by welding, resistance to thermal shock between room temperature and cryogenic temperatures and freedom from fragilization at these temperatures, these straws are extremely reliable for the cryogenic storage of rare or dangerous biological liquids such as viruses and the products of genetic engineering.
However, it has been found that ionomer resins, especially those with particularly attractive properties at cryogenic temperatures, are more difficult to print on than materials used for manufacturing straws used on a large scale (such as the straws used for artificial insemination of cattle and sheep), the constituents of the printing inks used diffusing less readily into ionomer resins, with the result that the printed characters are not so clear and more easily rubbed off.
Furthermore, even if these ink constituents diffuse less readily into ionomer resins, there remains the risk that diffusion of solvent through the thin wall (which is only a few tenths of a millimeter thick) could interfere with the biological liquid or that diffusion of solvent into the thickness of the wall could compromise the properties of the ionomer resin at cryogenic temperatures.
When the rarity of the biological liquid samples or the associated level of hazard make it necessary to reduce the number of straws containing the same biological liquid, it is desirable to make the straws more distinctive to reduce the frequency of errors through making the wrong choice. All the more so in that one effect of cryogenic storage is to cause the straws to be covered with frost as soon as they are removed from the liquid nitrogen, so that they are difficult to identify from the printed markings alone.